A document feeder 92 as shown in FIG. 20 is provided on the top portion of the main body of, for example, a copying machine 98 which is capable of copying two sides of a sheet of paper. In this document feeder 92, as shown in FIGS. 21 and 22, a document E placed on a document tray 95 is transported toward a reversing roller 96 by a plurality of transport rollers 93 and a conveyer belt 94. As shown in FIGS. 23 through 25, the document E which has been turned over by the reversing roller 96, is then placed by the rotation of the conveyer belt 94 at a predetermined position on a glass plate 99 shown in FIG. 26. In this position, the document E is subjected to scanning through the glass plate 99 by light projected from a light source, not shown.
Next, the document E having been subjected to the scanning light as described above, is turned over again by the reversing roller 96 as shown in FIG. 27, and then, as shown in FIGS. 28 and 29, is placed by the rotation of the conveyer belt 94 to the predetermined position on the glass plate 99 shown in FIG. 30. At this time, the side of the document E to be scanned is reversed to that of the former position shown in FIG. 26.
Then, placed in this position shown in FIG. 30, the document E is again subjected to scanning by light projected from the light source, not shown. Thereafter, as shown in FIG. 32, the document E is discharged onto a document discharge tray 97 located above the conveyer belt 94 by the reversing operation of the reversing roller 96 as illustrated by FIG. 31, and as such a two-sided copying operation is completed.
Recently, there have been demands on those image processing apparatuses, such as copying machines, toward high-speed image processing. In order to achieve high-speed image processing, the document feeder is also required to provide high-speed handling of the documents.
However, in the conventional document feeder 92, the two-sided copying operation is conducted through a complicated sequence of processes with respect to the document E, such as: feeding.fwdarw.transporting.fwdarw.reversing.fwdarw.backward transporting.fwdarw.copying (one side).fwdarw.reversing.fwdarw.backward transporting copying (the other side).fwdarw.discharging. Thus, after placing the document E on the document tray 95, many processes including two reversing operations are necessary to complete the two-sided copying operation. Further, when the document E is to be scanned by light, it should be stopped exactly at the predetermined position on the glass plate 99.
Furthermore, in the conventional document feeder, the length of the document transport path and the timing of document feeding are designed to be suitable only for the largest size of the documents that the copying machine can handle. Consequently, in the case of feeding smaller-size documents, the time interval between the feeding operations and the length of the document transport path become longer than required.
This results in a drawback in which the average processing speed per document in the two-sided copying operation becomes considerably low.
Therefore, in order to increase the average processing speed of the document feeder with a view to achieving high-speed image processing in copying machines or other apparatuses, the momentary transporting speed of the document feeding mechanism including the conveyer belt 94 and reversing roller 96 should be increased, that is, the speed of the document feeding mechanism during operation should be increased.
However, the faster the momentary transporting speed of the document feeding mechanism becomes, the more damage the document suffers when a paper jam occurs. Further, as the momentary transporting speed is increased, more problems are caused in that the stability of document feeding is impaired, copied images may have a missing portion, or the number of paper jams may increase.